Acknowledgements I would like to thank Christian Blodau for the supervision and advice during all phases of this work. I would like to thank all members of the Hydrology department and all the assiduous student assistants and technicians for their help. Without their support, this work would not have been possible: Stefan Peiffer, Michael Radke, Marieke Oosterwoud, Beate Fulda, Martina Heider, Karin Söllner, Isolde Baumann, Jutta Eckert, Martina Rohr, Heidi Zier, Likke Likke, Diana Burghardt, Tobias Goldhammer, Tobias Heitmann, Markus Bauer, Marianna Deppe, Julia Beer, Jan Pfister, Björn Thomas, Tobias Biermann, Severin Irl, Niklas Gassen, Benjamin Kopp, Lukas Gudmundsson, Christine Mahler, Ireneusz Forys, Martin Back. I would like to thank Bruno Glaser for the opportunity to measure stable carbon isotopes in his laboratory; and Gerhard Gebauer and Stefanie Goldberg for analysis of isotope analysis in low concentration samples. I would like to thank the helpful coordinators and technicians of the Research Group FOR 562: Egbert Matzner, Gunnar Lischeid, Werner Borken, Gerhard Müller, Gerhard Küfner, Uwe Hell, Andreas Kolb. I would like to thank all people providing advice and helpful comments, particularly Kirsten Küsel, Marco Reiche, Markus Horn, Jörg Gelbrecht, Dominik Zak and many others. I would like to thank my family for the support during all phases of my studies. I would like to thank my wife Johanna for the help, love and trust. 1

Summary Northern peatlands cover only about 3 % of the land surface, yet they store approximately 30 % of the global soil carbon stocks. On the other hand, peatlands contribute about 3-10 % to the global methane burden into the atmosphere. Climate predictions foresee not only an increase in the global mean temperature, but also a considerable change in precipitation patterns. As peatlands critically depend on hydrological conditions, a change in precipitation intensities and distribution is likely to affect the carbon sink and source function of peatlands. Thus, these ecosystems have become the focus of an increasing number of environmental studies over the past decades, trying to elucidate the response of peatland elemental cycles and budgets to climate change induced disturbances. From these studies, a basic understanding of carbon and elemental cycles in peatlands and their controls has already been established. Temperature, water table levels, and nutritional status have been identified to be the key factors affecting carbon mineralization. Low water table levels, high temperatures, and a higher nutrient availability mostly increased respiratory activity, but reduced methane production and –emission. Existing studies, however, investigated changes in average environmental conditions in the long term, while the impact of extreme weather on peatland elemental cycles is still largely unknown. Moreover, most studies do not provide a mechanistic understanding of the redox processes underlying the response of peatlands to fluctuations of the water table level. Based on laboratory studies, a thermodynamically constrained competition of the different terminal electron accepting processes for common electron donors was postulated. In this concept, methanogenesis provides the least energy. A detailed validation of this concept under natural or near-natural conditions is, however, still lacking to date. Moreover, the processes that renew alternative electron accepting capacity during drought are still not yet understood, as well as re-oxidation of electron acceptors due to oxygen input by plants. Fens were also identified to be notable sources or sinks for arsenic. The close association of arsenic with the iron- and sulphur-dynamics – and thus likely redox dynamics during fluctuations of the water table level in general – is already known. Nevertheless, there exist hardly any study investigating arsenic dynamics and solid phase associations for fens. The main objective of this work was therefore to study the effects of more pronounced drying and rewetting events on redox processes of carbon, iron, and sulphur – and concomitantly arsenic – in an electron acceptor rich fen-ecosystem. Therefore, we conducted experiments in the laboratory, using intact soil monoliths and subjecting them to a drying and rewetting cycle under controlled conditions. We traced changes in the carbon surface fluxes as well as respiratory activity and turnover of electron acceptors within the soil. In a complementary field approach, we induced an intensified drought period and a subsequent heavy rain event, using a drainage system and a temporary roof construction. In contrast to some existing studies, we could not find a notable effect of the drying/wetting treatment on the overall carbon budgets of the peat in this study. There was an obvious effect of 6 SUMMARY

drying/wetting on respiration within the soil, increasing drastically during drought, but the net carbon budget was by far dominated by the autotrophic activity of the vegetation (55-65 %) which was hardly affected by the treatment. Due to the drought event, methanogenesis was effectively suppressed in the unsaturated part of the profile and re-established after rewetting only after a notable time lag of some weeks. This suppression of methanogenic activity – in the laboratory and in the field approach – could successfully be explained by a reoxidation of reduced iron and sulphur compounds, providing alternative electron accepting capacity during and after drought. This reoxidation of reduced species could be identified in solutes and solids. Only after depletion of alternative electron acceptors, methanogenic conditions could re-establish in the entire profile. Locally, however, in micro-environments especially in the uppermost, intensively rooted layers, methanogenesis re-established even before alternative electron acceptors had been depleted. Based on the obtained data, we propose the high availability of easily degradable organic material, a still high water content, and poor aeration of the peat to responsible for this observation. These factors could support a local depletion of alternative electron acceptors, and thus establishment of anaerobic conditions so that methanogenesis could occur in locally distinct micro-environments. The analysis of the isotopic composition of the dissolved CO and the methane produced suggested that the methane was formed via the CO -2 2reduction pathway with H as the electron donor. This pattern was not affected by the drying/wetting 2treatment as the methane formed after rewetting showed the same isotope fractionation factors as observed before drought. Exceptionally high isotope fractionation factors suggested thermodynamic conditions to be quite unfavourable for methanogens. This coincided with the observation that most of the peat was likely structured in small micro-environments of locally distinct redox conditions, allowing a rapid switch between methanogenic and methanotrophic conditions on a scale smaller than the sampling devices used. The arsenic dynamics under variable redox conditions generally followed the dynamics of ferrous iron, especially in the intensively rooted uppermost soil layers. Coincidingly, a major part of the arsenic was found in the reactive iron-hydroxide fraction, readily available for microbial reduction. Although the total arsenic content in the solid phase was comparably low in the fen under study, -1concentrations of arsenic in the pore water ranged from 10 – 300 μg L and thus exceeded common drinking water standards mostly by far. Methylated arsenic species did not play a noteworthy role in this fen and the immobilization of arsenic in sulfidic phases during reducing conditions was also negligible when compared to mobilization from iron-hydroxide reduction. This study clearly demonstrated the importance of the – although shallow – unsaturated zone of fens for the carbon turnover within the soil. The high availability of labile organic matter – provided by the vegetation – allowed for reductive processes in these layers, including methanogenesis, but structured on a small aggregate scale. For the overall carbon budget of the fen ecosystem, however, autotrophic activity was most important, which was hardly affected by the experimental manipulation.